The present invention relates to a semiconductor laser device.
A high-output ultrashort-pulse semiconductor laser device that is formed of a GaN-based compound semiconductor and has a light emission wavelength in a 405 nm band is expected, as a light source of a volume-type optical disk system that is expected as a next-generation optical disk system following a Blu-ray optical disk system, a light source necessary in fields such as a medical field and a bioimaging field, etc. As a method to generate short-pulse light in the semiconductor laser device, mainly, three types of methods, that is, gain switching, self-pulsation, and mode locking are known. The mode locking is further classified into active mode locking and passive mode locking. In order to generate a light pulse based on the active mode locking, it is necessary to configure an external resonator with the use of a mirror, a lens, etc. and to further perform radio-frequency (RF) modulation on the semiconductor laser device. On the other hand, in the passive mode locking, a light pulse is generated by a simple direct-current drive by utilizing self-pulsation operation of the semiconductor laser device.
In order to cause the semiconductor laser device to perform the self-pulsation operation, it is necessary to provide a light emission region and a saturable absorption region in the semiconductor laser device. Here, the semiconductor laser devices are classified, based on an arrangement state of the light emission region and the saturable absorption region, into a SAL (Saturable Absorber Layer) type, a WI (Weakly Index guide) type, etc. in which the light emission region and the saturable absorption region are arranged in a vertical direction and a multi-electrode type in which the light emission region and the saturable absorption region are arranged side by side in a resonator direction. The multi-electrode-type semiconductor laser device is known, for example, from Japanese Unexamined Patent Application Publication Nos. 2004-007002, 2004-188678, and 2008-047692. It is considered that a GaN-based semiconductor laser device of the multi-electrode type has a larger saturable absorption effect, and generates a light pulse with a narrow width, compared to a SAL-type semiconductor laser device.
As an embodiment of the GaN-based semiconductor laser device of the multi-electrode type,
a semiconductor laser device including:
(a) a laminate structure in which
a first compound semiconductor layer having a first conductivity type and being formed of a GaN-based compound semiconductor,
a third compound semiconductor layer formed of a GaN-based compound semiconductor, the third compound semiconductor layer configuring a first light emission region, a second light emission region, and a saturable absorption region that is sandwiched by the first light emission region and the second light emission region, and
a second compound semiconductor layer having a second conductivity type that is different from the first conductivity type and being formed of a GaN-based compound semiconductor
are laminated in order;
(b) a strip-like second electrode formed on the second compound semiconductor layer; and
(c) a first electrode electrically connected to the first compound semiconductor layer, wherein
the laminate structure has a ridge stripe structure,
the second electrode is configured of a first portion, a second portion, and a third portion, the first portion causing a forward bias state by applying a direct current to the first electrode via the first light emission region, the second portion causing the forward bias state by applying a direct current to the first electrode via the second light emission region, and the third portion applying an electric field to the saturable absorption region,
the first portion of the second electrode is separated from the third portion of the second electrode by a first separation groove,
the second portion of the second electrode is separated from the third portion of the second electrode by a second separation groove, and
laser light is emitted from an end face on a second light emission region side of the semiconductor laser device is known.